1. Field of the Invention
The present invention relates to a semiconductor device, particularly, a thin semiconductor device provided with a large number of pins, said device permitting an efficient dissipation of heat generated within a semiconductor chip, and to a method of manufacturing the same.
2. Description of the Related Art
FIGS. 1 to 3 show conventional semiconductor devices provided with a large number of pins and having dimensions x=3.6 mm and y=32 mm. In the device shown in FIG. 1, a semiconductor device of a TAB structure is bonded to a lead frame and covered with a molding resin. To be more specific, the semiconductor device shown in FIG. 1 comprises a carrier tape 1, an island 2, a semiconductor chip 3, a lead frame 4, and a molding resin 5. The conventional device of this type is defective in that the island 2 has a small area, with the result that the heat generated in the semiconductor chip 3 is not dissipated efficiently.
A heat spreader structure of QFP is employed in the conventional device shown in FIG. 2. The semiconductor device of this type comprises a semiconductor chip 3, a lead frame 4, a molding resin 5, a heat spreader 6, a wire 7 and an adhesive 8. The device shown in FIG. 2 has a heat resistance lower than that in the device shown in FIG. 1.
However, the conventional devices shown in FIGS. 1 and 2 leave much room for further improvement. First of all, when it comes to, particularly, a semiconductor device provided with a large number of pins, the chip size is determined by the pitch of aluminum electrodes formed on the semiconductor chip. It should be noted in this connection that the semiconductor chip 3 is connected to the lead frame 4 by the wire 7, with the result that the area other than the semiconductor chip acting as an IC is increased, leading to an enlargement of the chip size and, thus, to an increased manufacturing cost.
A second problem to be noted is that the molding resin (enclosure) tends to be cracked in the step of mounting the semiconductor device to a printed circuit board by reflow. It should be noted that the bonding between the molding resin 5 and the heat spreader 6 is poor, with the result that water enters the clearance between the molding resin and the heat spreader. The water entering the particular clearance is evaporated by the heat in the mounting step of the semiconductor device to the printed circuit board so as to bring about the cracking noted above.
A third problem is that a satisfactory heat dissipation cannot be obtained in the conventional device because the semiconductor chip 3, the island 2 and the heat spreader 6 are surrounded by the molding resin 5. Incidentally, if the area of the heat spreader 6 is enlarged, the crack of the molding resin, i.e., the second problem noted above, is promoted. On the other hand, if the area of the heat spreader 6 is diminished, the heat dissipation is rendered poor.
What should also be noted is that the process for manufacturing the conventional semiconductor device necessitates the step of cutting a dam bar mounted to the lead frame before preparation of the final product (fourth problem). Incidentally, the dam bar is mounted for preventing a resin leakage because the resin layer is formed by the transfer molding.
The conventional device shown in FIG. 3 is of an M Quad structure. This device comprises a metal plate 31, an island 32, a semiconductor chip 33, a lead frame 34, an epoxy series adhesive 35, and a metal cap 36. The device shown in FIG. 3 is not covered with a molding resin. In addition, the metal plate 31 has a large area. It follows that the device exhibits a high heat dissipation. However, the metal plate 31 widely differs from the semiconductor chip 33 in the thermal expansion coefficient. As a result, a severe thermal fatigue is brought about when the device is subjected to heat cycles, giving rise to crack occurrence in the semiconductor chip 33. In other words, the metal plate 31 is not provided with any means for moderating the strain caused by the thermal expansion, giving rise to the crack occurrence. What should also be noted is that the metal plate 31 and the island 32 are bonded to each other with the epoxy series adhesive 35 interposed therebetween, with the result that the heat generated within the semiconductor chip 33 is not necessarily dissipated satisfactorily.